1. Field of the Invention
The present invention relates to a step motor driving circuit for driving step motors provided with pairs of phase windings magnetically coupled.
2. Description of the Prior Art
The step motor driving circuits must generally fulfill several requirements. One requirement is to obtain high torque for different rotation speeds. Another requirement is to achieve a high system efficiency (that is, cutting of power losses relative to the active power produced). It is known that the torque available on the step motor shaft is proportional to the inductance of the motor winding and to the current flowing into the windings. Such torque decreases as the rotation speed of the motor increases. In fact, during static conditions some preestablished phase windings are continuously energized. When the motor is caused to rotate, its phase windings are periodically and selectively energized in relation to the requested speed. Accordingly during static conditions the value of the current flowing into the phase windings is limited only by the resistance of such windings. Under dynamic conditions the current is further limited by the inductance of the phase windings, and varies with time according to a time constant .tau.=L/R where L and R are respectively the inductance and the resistance of the phase windings. The maximum current value which is reached in the energized phase windings is therefore conditioned by the energization period; i.e., by the energization frequency. In case of very low energization frequencies and therefore of very long energization periods, longer than the time contant .tau., the current flowing into the energized phase windings may reach values very close to the steady state currents obtainable in static conditions. In case of high energization fequencies, and therefore of energization periods shorter than the time contant .tau., the current flowing into the energized phase windings reaches maximum values which are lower than the steady ones; in particular the higher the energization frequency, the lower such current values are.
In order to increase the torque delivered by a step motor at high energization frequencies, the energization current must rapidly rise when the phase windings are energized and rapidly fall to zero when the phase windings are de-energized. This can be obtained in several ways. One solution is to modify the characteristics of the circuits including the windings in such a way as to reduce the related time constant. For instance an additional resistance may be connected in series to the windings and, at the same time, the supply voltage may be increased in order to maintain the steady value of the energization current unchanged. Such a solution, however, reduces greatly the system efficiency because of the power dissipated by the additional resistance. Additionally there are technological and economical limits in the use of high supply voltages. In order to avoid a degradation of the system efficiency, the supply voltage may be increased, although within certain limits, without using additional resistances. A solution of such kind involves the use of control circuits which maintain the current flowing into the energized windings within preestablished limits. The most used energization current control circuits are the ones disclosed in U.S. Pat. Nos. 4,107,593 and 3,812,413.
Another solution is to feed the windings with two voltages: a high voltage used during the initial energization period and a lower voltage used successively to hold the current to a predetermined value. Such a solution is described in U.S. Pat. No. 3,659,176. In U.S. Pat. No. 4,253,052 an additional solution is suggested and described. According to such patent the energy stored in a winding which has to be demagnetized is transferred, through an auxiliary inductor, to a capacitor connected in series to a phase winding to be subsequently energized. Each phase winding is provided with a capacitor connected in series to the supply voltage and to the same winding. When the winding is energized, the supply voltage is therefore the sum of the power supply voltage and of the capacitor charging voltage. Such supply voltage decreases as the capacitor discharges. The magnetic energy stored into an energized winding is therefore recovered and used to energize successively another winding. This approach is embodied by means of very complex control circuits and may be only used if the several motor windings have both their ends accessible and not directly connected.
In the last-mentioned U.S. patent, with reference to FIG. 4, a simpler solution is also disclosed, but it may be used in "full stepping operation" step motors. According to such simpler solution the energy stored in an energized phase winding is transferred, as soon as such winding is deenergized, to a capacitor coupled to a different phase winding and no additional inductor is used. Such a solution cannot be used in step motors provided with chopping current control systems, unless devices limiting the capacitor charge are provided. Moreover it requires that a capacitor and a by-pass diode be coupled to each phase winding. Furthermore, the demagnetization of the winding which is de-energized is relatively slow because it takes place with a demagnetization voltage which varies according to the charging voltage of the capacitor; i.e., from 0 to the maximum charging voltage reached by the capacitor.
These problems are overcome by the step motor driving circuit of the present invention which is suitable for step motors provided with pairs of magnetically coupled windings.